The invention relates to microwave intrusion detection systems, especially to improvements which increase the signal to noise ratio thereof and reduce the likelihood of false alarms.
Perimeter intrusion detection systems, such as those described in U.S. Pat. Nos. 4,328,487 and 4,358,764, which are assigned to the present assignee and are incorporated herein by reference, are finding increased application. Such systems are often used in connection with a fence or wall that bounds the area to be protected. The types of systems include a coaxial cable or transmission line with a microwave frequency transmitter at one end and a receiver at the other end and a plurality of spaced antennas loosely coupled to the cable and aimed into the protected region. This type of system is especially useful in irregular terrain because the cable and antennas can follow the contours of the terrain. One of the disadvantages of prior systems of this type is that the target reflection signal is very small compared to the reference signal that is received by the receiver via the cable or transmission line. Noise generated by the oscillator and noise generated by the connectors due to forces, such as wind forces, on the transmission line falls within the doppler pass band and often results in false alarms. The target reflection signal is very small relative to the reference signal because the entire microwave signal, typically approximately 915 megahertz, transmitted along the cable by the transmitter is modulated at a roughly 10 to 50 kilohertz rate and only a small proportion of the microwave signal energy is transmitted by a particular antenna, and still less is reflected.back from a moving target and coupled back onto the cable. (Such modulation or chopping of the carrier signal is done in order to avoid the l/f low frequency noise amplification problems that would otherwise be encountered, as those skilled in the art will recognize). Thus, it is very difficult to recover the doppler frequency signal due to the very low signal to noise ratio.
Therefore, it can be seen that there is an unmet need for an intrusion detection system and method of the above-described type which avoids false alarms due to noise generated by the transmitter and by the connectors due to wind forces on the transmission line.
Accordingly, it is an object of the invention to provide an improved microwave intrusion detection system including a plurality of antennas loosely coupled along a transmission line wherein reflected signals from targets within the protected region, received by the antennas and conducted along the transmission line to a receiver, are reliably detected despite the presence of a relatively high level of connector noise in the doppler pass band.
It is another object of the invention to provide relatively inexpensive circuitry in a microwave intrusion detection system which reliably detects moving targets in the protected region despite the presence of a relatively high level of connector noise and/or transmitter noise in the doppler pass band.